When trying to "Add a new SSL certificate" using App Engine's Settings tab that was generated with Let's Encrypt via Google App Engine's console results in a dialog error and a 400 response to the POST request.
Error
"The SSL certificate provided could not be inserted."
A previously generated (about 2 months ago - not yet expired of course) SSL key/certificate via the exact same method is inserted just fine - but any newly generated one does not. I attempted both traditional Let's Encrypt and the relatively new Certbot method. Also tried multiple subdomains, naked domains, singular domains and each results in the same error.
I've seen several people spec that --rsa-key-size 2048 solved the same issue, but I've tried specifying that as well (even though it is the default for Certbot as is). Other answers have been "waiting 2 hours and now its working" - looking for a real solution as unreliable inserts and expired certs can become a real pain.